Flat-rate water bills remove the incentive to conserve. When your bill stays the same no matter how much you use, you're less likely to turn off taps or run smaller loads. Usage-based billing ties what you pay to what you use, which actually makes people think about water and change their habits.
Chapters
If you know that your bill is not going to change, your water use is going to change, meaning you're not going to care whether you turn off the sink while you're brushing your teeth, or you do a load of laundry with 2 pieces of clothing in there, or run the dishwasher with 4 dishes in there. It's only when that bill fluctuates based upon the property's usage that does that actually modify the behavior of the resident. You want the resident to think that my usage and my consumption is going to change my bill. But doing it as a flat rate, although it sounds appealing as an owner because it sounds easy, you're actually not incentivizing your residents to conserve. Not only do you want your residents to conserve, that brings the property bill down, but it allows them to start taking accountability for their usage and modifying their behavior because of that.